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A planet orbiting the Sun is moving sideways rather than towards the Sun. Other things being equal, you would expect it actually to move off sideways and rapidly to distance itself from the Sun. But the Sun keeps hauling it back in because of gravity. The gravitational force exerted by the Sun on the planet attracts the planet towards the Sun. The reason that the planet doesn't then move closer to the Sun is because its tendency to move away sideways just counteracts the Sun's attraction.